In general, most of mobile small-sized secondary battery markets are occupied by lithium ion batteries and lithium polymer batteries.
The lithium ion batteries and the lithium polymer batteries have high voltages and high energy densities as compared with other batteries. Among them, in the lithium ion battery, a lithium metal oxide is used at a positive electrode and a carbon metal is used at a negative electrode.
Further, the lithium ions of the battery move between the positive electrode and the negative electrode while the battery is charged or discharged.
The mobile secondary battery may be embedded in a device or may be detachably mounted on the device.
The battery chargers for charging the batteries mostly employ a constant current-constant voltage (CC-CV) charging method. In the method, a battery is charged with a constant current for a predetermined period of time and then is charged with a constant voltage if the level of the battery reaches a charging level of a constant voltage.
Most of the rapid charging systems terminate charging at a time point when the battery reaches a constant voltage (CC-CV).
At this time, the charger is generally switched to a charging operation of a low current.
A charging operation of a lower current varies based on an SOC of the battery. Then, the charging speed of the lithium ion battery is adjusted such that the voltage of the battery does not exceed a specific voltage value.
This is known as a constant voltage charging operation of a CC-CV profile.
The lithium ion battery typically is charged by using a CC-CV method.
The CC-CV method algorithm charges the battery up to a specific voltage with a fixed current. Once the voltage of the battery reaches a specific voltage, the charger is switched to a charging current of a low speed. Thereafter, the specific voltage for charging generally is selected by manufacturers.
However, in the conventional rapid charging system using a CC-CV method, as the battery degrades rapidly due to the charging efficiency and the heat emitting reaction of the battery when a current for static current charging is high, the battery manufacturers mostly use charging of a low current (5 W).
However, because the sizes of the displays for a smartphone are increasing from 3 inches to 5 inches while consuming high energy and multi-tasking services and LTE, 3G, and Wi-Fi communications are multiply used, necessary energy increases so that the smartphone users have to frequently charge the batteries.
The currently commercialized mobile devices take a minimum of about two hours to completely charge the battery packs. Further, because power users charge the batteries two or three times after charging the battery, rapid charging is required.
Because a high current is applied for the rapid charging as compared with general charging, the intercalation and deintercalation speeds of the lithium ions in the electrodes cannot sufficiently follow the applied current. Accordingly, the speed of a side reaction that degrades an electrode material becomes higher so that the resistance of the lithium secondary battery increases, and the temperature of the battery excessively increases during a charging operation so that the cycle life span of the lithium secondary battery rapidly decreases.
Accordingly, studies on an optimum charging condition for reducing an increase of temperature of the lithium secondary battery and shortening the charging time of the lithium secondary battery are inevitably necessary to maximize the life span of the lithium secondary battery in all fields in which the lithium secondary battery is used.
FIG. 1 is a picture easily expressing an influence of factors that influence reduction of a performance of a battery.
Referring to FIG. 1, it is safe to smoothly control a temperature at which an electrode material degrades during charging of a battery.
However, if the temperature of the battery is unstable for degradation, a charging state becomes unstable and an overcharging or over-discharging operation may occur.
Further, the degradation increases the resistance of the lithium secondary battery, and excessively increase the temperature of the lithium secondary battery during a charging operation so that the cycle life span of the lithium secondary battery rapidly decreases.
Accordingly, studies on an optimum charging condition for reducing charging time while decreasing an increase of temperature are essential to maximize the life span of the lithium secondary battery in the actual field of chargers.
The constant current-constant voltage (CC-CV) charging method that has been generally adopted at the time when the lithium ion batteries are common shows a low reduction of capacity, a short charging time, a convenient operation, and a low internal resistance for a long life span. However, when the CC-CV charging method is adopted, unsafety occurs at a positive electrode and a negative electrode of the interior of the battery.
When an artificial graphite negative and an Li positive electrode are selected, lithium plating occurs in most charging conditions, in particular, at a high current and a low temperature. Even in the CC charging range, a potential of the graphite negative electrode decreases to below 0 V. While a process of reinserting plated lithium into graphite occurs together with lithium plating, a low charging capacity is shown.
When the current exceeds a specific level, the increase of current cannot reduce charging time any further and a CV charging time becomes longer and the lithium plating deteriorates as well. Moreover, the decrease in the temperature of the battery further deteriorates the lithium plating.
In the CC-CV charging, after the voltage of the battery reaches an upper limit voltage (4.1 V to 4.2 V) with a constant current, a CV condition is maintained until the current of the battery reaches a preset low current value. Then, the CV condition may seriously extend the charging time. That is, the diffusion of the lithium ions in the electrode during charging causes a rate limiting step, and concentration polarization is essentially caused as the lithium ions are diffused for a long time. As the voltage of the battery reaches an upper limit voltage during rapid charging, the current may be lowered to a preset limit value before the active material is completely consumed.
The charging time of the lithium ion battery of more than two hours may be relevant to the safety and life span of the battery, and it is known that if the charging time exceeds two hours, a side reaction occurs and stability of the battery deteriorates due to the degradation of the battery.
In order to solve this problem, as an optimum charging method has been required to secure rapid charging and cycle stability of the lithium ion battery, a multistage constant current-constant voltage (MCC-CV) charging method has been developed.
The lithium battery is related to an internal resistance (Ohmic polarization) of the battery and a polarization phenomenon associated with movements of charges between the interfaces of the electrode and the electrolyte during charging. An overvoltage is caused due to the internal resistance of the battery and the active polarization, the conductivity of the ions is lower than the conductivity of the electrons, impurities may be contained in the electrode material, and a difference between the concentrations of lithium ions for electrodes may be caused due to the difference between the lithium ion diffusion speeds on a surface and the interior of the electrode material, which in turn causes a polarization phenomenon.
In the battery for a mobile device, when the current of the battery is higher than an existing current in a CC-CV (constant current-constant voltage), it influences aging and stability of the battery due to the lithium plating and the side reaction, and the charging efficiency of the SOC (a method of predicting a charging state with the voltage of the battery) based on voltage is high, but shows a high value due to the increase in the voltage due to the actual charging current, and as the actual charging amount corresponds to a voltage increment due to heat emission and current, the battery has a charging efficiency that is lower than the actual value in the actual OCV.
Further, in the battery for a mobile device, temperature greatly influences charging.
In particular, in the case of a high temperature of 60° C. or higher, aging of the battery accelerates and the life span of the battery rapidly decreases, and even in the case of a low temperature, lithium plating rapidly occurs during rapid charging.
Accordingly, it is necessary to optimize a charging method based on temperature.